In physically and chemically demanding environments, such as oil patch and gas turbine applications, there is a need for higher strength nickel-base alloys having corrosion resistance greater than the workhorse 3% molybdenum precipitation hardened alloys--INCONEL.RTM. alloy 718 (UNS N07718) and INCOLOY.RTM. alloy 925 (UNS N09925). In particular, a yield strength in the range of about 140-170 ksi (965-1172 MPa) combined with superior corrosion resistance is desired by fabricators and component manufacturers. The "UNS" prefix and the "UNS" numbers set forth herein refer to the alloy compositions in the well-known "Unified Numbering System" established by SAE HS-1086 and ASTM DS-566.
Oil patch applications include subsurface and well head completions and drill components. High strength and corrosion resistant containment rings and associated components on gas turbine engines require lightweight but robust construction.
Age hardenable alloys based upon nickel and containing precipitation hardening amounts of titanium, niobium and/or aluminum have been known and used for many years. Various heat treatment techniques have been employed to obtain desired physical and chemical characteristics. See, for example, U.S. Pat. No. 3,871,928.
More particularly, component fabricators and designers have identified the following characteristics and targets as desirable for specific oil/gas and turbine applications:
(1) Age-hardenable yield strength .gtoreq.140 ksi (968 MPa); PA1 (2) Low temperature Charpy V-notch impact strength at -75.degree. F. (-58.degree. C.)=25 ft-lbs (111 N-m); PA1 (3) Pitting resistance superior to alloy 718 (UNS N07718) and alloy 925 (UNS N09925); PA1 (4) Resistance to hydrogen embrittlement per NACE TM-0177 test; PA1 (5) Stress corrosion cracking resistance to moderately sour oil field environments at temperatures from 250.degree. to 350.degree. F. (121.degree. to 177.degree. C.); PA1 (6) Fracture energy as expressed by tensile strength elongation greater than exhibited by alloy 718 (UNS N07718); and PA1 (7) High temperature strength greater than exhibited by alloy 625 (UNS N07716).
The typical commercial compositions of alloy 725 (UNS N07725) and alloy 625 (UNS N07716) are given in Table 1 below:
TABLE 1 Chemical Composition (wt. %) Alloy 725 Alloy 625 (UNS N07725) (UNS N07716) Nickel 55.0-59.0 57.0-63.0 Chromium 19.0-22.5 19.0-22.0 Molybdenum 7.0-9.5 7.0-9.5 Niobium 2.75-4.0 2.75-4.0 Titanium 1.0-1.7 1.0-1.6 Aluminum 0.35 max. 0.35 max. Carbon 0.03 max. 0.20 max. Manganese 0.35 max. 0.20 max. Silicon 0.20 max. 0.20 max. Phosphorus 0.0l5 max. 0.0l5 max. Sulfur 0.0l0 max. 0.0l0 max. Commercial Impurities Trace Trace Iron Remainder Remainder
Alloy 725 (UNS N07725) is strengthened by precipitation of double gamma prime phase during an aging treatment. Before aging, the alloy is currently solution annealed at 1900.degree. F. (1040.degree. C.) and air cooled or water quenched. For sour gas applications, the published recommended aging treatment is 1350.degree. F. (730.degree. C.)/8hours, furnace cooled and then air cooling.
In summary, in order to obtain the published high yield strength for, say, age hardened rounds (133 ksi (917 MPa)) or strip (143 ksi (992 MPa)), the current practice is to anneal, cold work and then age.
In order to exceed the properties of alloys 718 and 925, it was contemplated that a new heat treatment paradigm would be necessary.